Performance of 18 F-FAPI PET/CT in assessing glioblastoma before radiotherapy: a pilot study

Background We aimed to determine the performance of 18 F-FAPI PET/CT used for preprocedural assessment of glioblastoma before radiotherapy. Methods Twelve glioblastoma patients having undergone incomplete surgical resection or biopsy were examined with 18 F-FAPI PET/CT and MRI scanning before radiotherapy. All patients had confirmed tumor residues according to findings of histopathological and/or long-term clinical and radiological follow-ups. Lesion characterization data, including SUVmax and tumor-to-background ratio (TBR) on PET/CT were attained. PET/CT and MRI findings were compared in terms of number of lesions. The correlation between immunohistochemistry, molecular expression, and PET/CT parameters was also evaluated. Results 18 F-FAPI PET/CT detected 16 FAPI-avid out of 23 lesions in 12 patients described on MRI. MRI was statistically different from 18 F-FAPI PET/CT for lesion detection according to the exact McNemar statistical test (P = 0.0156). The SUVmax and TBR of the glioblastomas was 7.08 ± 3.55 and 19.95 ± 13.22, respectively. The sensitivity and positive predictive value (PPV) of 18 F-FAPI PET were 69.6% and 100%, respectively. Neither the Ki-67 index nor the molecular expression was correlated with the FAPI-PET/CT parameters. Conclusion 18 F-FAPI PET/CT detects glioblastomas at a lower rate than MRI. However, the 100% PPV of the examination may make it useful for differentiating controversial lesions detected on MRI. The 18 F-FAPI-avid lesions are displayed more clearly probably due to a higher TBR. 18 F-FAPI PET/CT imaging might find application in glioblastoma biopsy and radiotherapy planning.


Background
Glioblastoma is reported to be the most prevalent primary brain malignancy in adults, which represents one of the most rapidly growing glial tumors, accounting for about 57% of glioma. It is associated with a worse prognosis and a high recurrence rate [1]. Only some 6.8% of glioblastoma patients survive beyond five years [2]. As the most common non-invasive imaging technique to assess glioblastoma [3,4], magnetic resonance imaging (MRI) has been employed as a diagnostic instrument for glioblastoma diagnosis, prognostication, and monitoring of therapy response [5][6][7][8][9]. Other methods, such as diffusion-weighted imaging (DWI), apparent diffusion coefficient (ADC), perfusion-weighted imaging (FWI), and magnetic resonance spectrum imaging (MRS) are also widely used in addition to conventional sequences [4,6,7,[10][11][12][13].
Positron emission tomography (PET) has been reported to provide additional information, which is helpful for identifying biological characteristics of tumor, differential diagnosis, and tumor size delineation and for an oncologist to plan a surgery, radiotherapy, and/or post-treatment tumor surveillance [14][15][16][17]. 18 F-fluorodeoxyglucose ( 18 F-FDG) is a commonly used PET tracer in clinical settings, which is known for diminished sensitivity due to increased physiologic uptake in the normal brain tissue [18]. When compared with glucose ( 18 F -FDG), amino acids including 11 C-MET, 18 F-FET, 18 F-FDOPA may be better tracers because they have higher specificity and lower signal/noise ratio [14,17,19].

Patients
Patients with glioblastoma who underwent assessment before radiotherapy in our department between August 2020 and March 2022 were enrolled in this study. The diagnoses were confirmed by biopsy or incomplete surgical resection plus histological study before imaging and/ or long-term clinical and radiological follow-up. The immunohistochemistry and molecular expression, such as Ki-67 proliferation index (Ki-67 index), O6-methylguanine-DNA methyltransferase (MGMT) promoter methylation status, Isocitrat Dehydrogenase (IDH) and telomerase reverse transcriptase (TERT) status, were collected. All patients were scanned with 18 F-FAPI PET/CT and MRI. The median time between the examinations was 3.5 days (range, 1 to 7 days).

PET/CT imaging
The patients were not specially prepped on the day of 18 F-FAPI PET/CT scanning. A Biograph mCT-64 scanner (Siemens, Germany) was used. The scanning was performed about 60 min after intravenous injection of 18 F-FAPI (0.12 millicurie/kg). The localizer was positioned with a scout head view. Low-dose CT (120 kV/110 mA) was then performed for anatomical localization and attenuation correction. Single-bed emission scans were obtained in 3-dimensional mode (acquisition time, 3 min). Reconstruction of data was done using an ordered subset expectation maximization iterative reconstruction algorithm (three iterations, 21 subsets). The emission data were corrected for random, scatter, and decay.

Image analysis
Two physicians with ten years of experience in brain tumor imaging evaluated the MRI images. Two nuclear medicine physicians who were experienced in PET/CT image assessment evaluated the PET/CT images. Disagreements were resolved by consensus. An abnormal focus of increased 18 F-FAPI uptake as compared with the background activity in the brain parenchyma was defined as positive on PET/CT. Standardized uptake values (SUV) were quantified using regions of interest (ROI). Should an area be found with increased uptake compared with the surrounding parenchyma, we drew a circular ROI over it where cystic or necrotic portions of the lesion was identified and avoided. Additionally, we drew an identical ROI over the contralateral normal cerebral cortex as a background. Tumor-to-background ratio (TBR) was calculated as the SUV max of the lesion divided by the SUV max of the contralateral normal cortex. The Response Assessment in Neuro-oncology (RANO) guidelines were used as the MRI reference. Lesions that were Gd-DOTA-enhancing, predominantly hyperintense with surrounding edema on the FLAIR sequence, or with a CHO/NAA ratio > 1.5 were considered positive on MRI images.

Statistical analysis
We performed the statistical analyses using SPSS (version 22.0; IBM). Continuous variables are expressed as mean ± SD and categorical variables as numbers and percentages. The number of positive lesions was compared using exact McNemar statistical test. Inter-parameter correlations were calculated with the Spearman test. P < 0.05 was statistically significant.

Patients' characteristics
Twelve patients (aged 34-73 yrs, average 52 yrs) with glioblastomas confirmed by incomplete surgical resection or biopsy were enrolled in this study before radiotherapy. The characteristics are displayed in Table 1. The Ki-67 index of the glioblastomas was 42.92 ± 21.58%.
Neither the Ki-67 index nor the molecular expression was correlated with the 18 F-FAPI-PET/CT parameters. (P > 0.05, Table 2)

Discussion
FAP overexpression and cancer associated fibroblasts (CAFs) found in tumor stroma are believed to cause the FAP-positive signaling in extracranial tumors [21,34]. Imaging techniques that target FAP are novel promising instruments for visualizing tumor stroma. Excellent performance of FAPI PET/CT has been reported in various tumors including glioblastoma [21,28].
Although fibroblasts do not exist in the brain, FAP-positive vessels in glial tumors, FAP-positive foci of neoplastic cells in gliomas, as well as FAP-positive stromal cells that function similarly to CAFs in epithelial cancers have been reported by several researchers [35][36][37][38]. FAP and FAP mRNA are overexpressed in most glioblastomas, especially in the mesenchymal subtype [37]. Some 65%  [37] or the status of the blood-brain barrier [19]. Although its sensitivity was not high, the PPV of 18 F-FAPI PET/CT was up to 100%, which may be useful for differentiating controversial lesions detected on MRI. However, prospective studies based on larger samples are still needed to validate the findings.
To this day MRI remains the most used imaging modality for glioblastoma diagnosis and evaluation of treatment response and prognosis [1,4,10]. A number of advanced MRI techniques such as functional MRI (fMRI), diffusion tensor imaging (DTI), and MRS have been adopted for tumor treatment including preoperative surgical planning or as an instrument to distinguish post-operative vascular damage from residual enhancing tumor [9,12,13,39]. However, it is highly challenging to assess sub-regions of tumors in MRI scans visually because of their anatomic complexity. This can be even more true in post-operative settings [13]. It is a time-consuming task for clinicians to delineate the targets and critical structures, where observer biases are probable. Up to 20% intra-and 28% interrater variability have been reported for determining glioma boundaries [40]. As a result, boundary delineation of radiotherapy target volume could be inaccurate. Advanced molecular imaging could be helpful at this point [41]. Although the lesion detection rate was lower on 18 F-FAPI PET/CT than on MRI in our study, the TBR of the lesion was high (range 7.90-30.21) and the boundaries were better visualized due to the low FAPI distribution in the normal cerebral tissues. FAPI PET/ CT shows potential for application in biopsy and radiotherapy planning for glioblastoma [28,30,42]. According to a Polish study, target volume delineation based on MRI and FAPI PET is differentiated [30]. Gross tumor volumes (GTVs) delineated using FAPI PET plus MRI is significantly higher compared with MRI alone [30]. Unfortunately, the metabolic GTVs were not calculated in this study due to the threshold value Fig. 2 The non-FAPI-avid lesions were confirmed by follow-up MRI (T2, FLAIR, CE-T1) 6 months later. A1-C1 were images of T2 sequence, A2-C2 were images of FLAIR, A3-C3 were images of enhanced T1. All enlarged and partially mixed signals were heterogeneously enhanced (short arrow, long arrow, and asterisk), indicating disease progression Table 2 Correlations between Ki-67 index, molecular expression, and FAPI-PET/CT parameters of MTV in FAPI PET/CT was still indeterminate. The further study is needed to analyze the different threshold value of MTV in FAPI PET/CT compared with the GTVs on MRI or RT planning. Ki-67 index is a quantitative measure of cell proliferation in histopathological assessment of glioblastoma and many other tumors [43]. Ki-67 index and TERT or IDH mutation are positively associated with overall survival of glioblastoma patients and a lower Ki-67 index and IDH-wildtype are linked with poor prognosis [43,44]. However, our findings failed to establish any correlations between the Ki-67 index or the molecular expression with the FAPI-PET/CT parameters, such as MGMT promoter methylation, TERT, and IDH mutation. Yet, only one of the glioblastoma patients in our study had IDH mutation. A larger sample may be needed in future research to further investigate our finding.

Limitation
Our sample size was limited with 12 patients and 23 lesions. As a pilot study, our findings are preliminary and should be interpreted with caution. Another limitation to our study was that some of the lesions were not labeled with FAP on immunohistochemistry. This prevented us from evaluating the correlation between FAP expression and the PET/CT parameters. Further studies with larger samples are needed to validate our results, especially the correlation findings.

Conclusion
Despite a lower lesion detection rate than MRI, the PPV of 18 F-FAPI PET/CT was 100%, suggesting it as a useful examination for indeterminate lesions detected on MRI. 18 F-FAPI PET/CT visualizes lesions more clearly with a high TBR, possibly because of lower FAPI distribution in the normal cerebral tissues. The technique may find applications in target volume delineation in radiotherapy of glioblastoma.